The Secret Underground Assassin Society
by goofythegoober
Summary: After a strange chain of events, an 18 year old Finn is "brainwashed" into becoming an assassin. He is now part of the The Secret Underground Assassin Society that works secretly underneath the Weaponry Kingdom(I made this up). Finn finds out that something big is going down, and sets his sights on stopping it while protecting the people he loves most. My First Fanfic. Please R & R
1. Chapter 1

Chapter One

Our story begins at a tree house. Not just your ordinary tree house however. This treehouse belonged to the two most well renowned heroes in all of OOO, Finn the human and Jake the dog. It was the still quite early in the morning, and yet, one occupant of this specific dwelling was not home.

"Dude!" Jake ejected from his sleep, "I had the craziest dream! I was there, and you were there, and, and… Finn?" Jake turned to look at his best friend, brother, and roommate's bed, just now realizing he was gone. Jake stretched himself out of bed and down the ladder to the next floor.

"Finn!? Fiiinn!?" Jake called throughout the house. "BMO, have you seen Finn?"

The small robot was currently completely submerged in a pile of dirty socks. After being addressed, BMO stuck his head out of the pile. "Oh yes, I saw Finn last night. I saw him on his way out. He told me to tell you that he would be gone for a while. He said he didn't know how long, just don't wait up. Help me put Finn's socks on," BMO said holding up two of Finn's dirty socks.

"Later, BMO," Jake said, slapping the socks out of BMO's hands. Jake sat down on the floor and simultaneously let out of a sigh. "Dang it Finn, not again."

Finn had been doing this for a while, slipping out early in the morning to do solo missions. He would be gone for days, sometimes weeks at a time out on these adventures and usually kept the details secret after he returned. This all started when Finn turned 18 four months ago, when he became more independent. It's not that they were emotionally dividing, they were as close as ever, it's just that there was not as much bro-time as before. They still went on the occasional endeavor as bros, but Finn was gone a lot of the time, making these a lot less common. With Finn gone, that also meant Jake had to find something to do with all the extra spare time. Jake would Usually just play video games with BMO. The lack of physical exercise rendered jake fatter and lazier than he used to be.

"It's okay Jake. I will never leave." BMO said, in order to try cheering up Jake.

"Yeah I know that," Jake said matter-of-factly, "It's just what if Finn gets donked up on one of these trips and I won't be there to help him."

"Do not worry. Finn is strong. And he knows Karate," BMO demonstrated a karate chop as an example of his own karate knowledge. BMO then held up two socks. "Which of these is better?"

"Yeah, I guess you're right," Jake said, ignoring BMO's question. He let out another sigh, "I wonder what Lady's doing?"

. . .

Finn the human stretched his arms above his head and yawned. He had gotten up extremely early because of an urgent message from the Weaponry Princess, the ruler of the Weaponry Kingdom. Over the past couple of years, multiple new kingdoms and empires appeared practically out of nowhere, the Weaponry Kingdom sprouting in the northwestern parts of OOO. Others include the Sports Kingdom in the far western parts, in which some sports tournament is always going on. Finn and Jake went to one just a few weeks back. There was also the paper kingdom where everything and everyone is made of paper which neighbors the candy kingdom. Finn once had to save the whole kingdom when it started raining without notice. Finn left with papercuts from head to toe, due to the people showing their appreciation through the form of hugs.

As Finn continued walking, he began to see the outline of the Weaponry Kingdom. This gave him the kick that he needed, like a shot of pure caffeine. When Finn first went to the Weaponry Kingdom, he instantly fell in love. The architecture, the people, the sheer number and craftsmanship of the weapons. There were more blacksmiths than any other occupation, and almost everyone knew how to use a sword. It was a weapon-using hero's paradise. This was Finn's second most visited kingdom, right beneath the Candy Kingdom, which Finn still pledged his allegiance to.

As Finn strolled into the kingdom walls, he was showered with greetings. Many of the usually stern faced guards gave him a salute and a smile as he passed. He had gained the respect of the whole military after defeating the captain of the guards in an arm wrestling match. Finn began to pass the many, thriving blacksmith shops. Although there were so many, and the residents of the kingdom didn't purchase from each other very often, they supplied weapons for the entire land.

Finn received waves from Greggor, the bowsmith, Nancy, who focused on magic weapons, and Jeoffrey, who made anything that included a chain, including nunchucks as well as many others, as he proceeded down the solid iron streets. The excess iron in the kingdom led to them being able to make things(sometimes unnecessary things) out of iron, one thing including the streets. This led to them being able to do architecturally amazing things.

Finn was turning a corner as a distant "help" was heard in the distance in the opposite direction, followed by a scream. Finn stopped dead in his tracks.

"What the…" Finn started. A second scream cut him off.

Finn didn't wait to hear a third. He began sprinting in the direction of the scream.

"Finn!" Randy the swordsmith tossed him a sword as Finn began picking up speed.

Without slowing or even stopping Finn caught the sword. "Thanks," he shouted over his shoulder as he strapped it across his back. Finn's last sword had broken during a battle against a sea giant. He still had yet to find a suitable replacement.

Finn rounded a corner into a semi-narrow alley that was dead slowed to a jog. A small crowd had gathered around something Finn couldn't see. Finn made his way through the crowd and made his way to what was being crowded around. A man laid there, with both legs broken and twisted at odd angles. He was lying in a pool of his own blood, and there was a gash across his throat. If Finn had to guess, somebody cornered this man. The man, as he backed away tripped and fell on his butt. As he was butt-scooting away from his attacker, said attacker stepped on and crushed both the man's legs. Then swiftly slit the man's throat before fleeing the scene. Finn dropped down on one knee and checked the man for a pulse, and when he found none, gave a moment of silence for the man.

Finn stood up and told the police, who had just arrived seconds ago what he believed happened.

"Thanks for the observations Finn," said the head officer, "We'll take it from here."

As he said that Finn caught a flicker of motion out of the corner of his eye. He turned and saw the head of a hooded figure on top of one wall in the alley.

"Hey, you up there!" Finn shouted,"Freeze!"

The head receded from the edge of the wall.

"Stop!" Finn shouted. With no time to waste, Finn began to scale the wall. When Finn emerged onto the roof, the cloaked figure was already two roofs ahead. "Damn it!" He said under his breath. Finn immediately started running.

 _Looks like I'm faster than whoever this person is,_ Finn thought. He was obviously gaining ground on the figure. Finn leaped from one roof to another. The figure was now only half a rooftop ahead and Finn was closing the distance further. The person reached inside their cloak and produce three throwing needles, and threw them at Finn, without slowing. Finn swiftly dodged them. They were close now. All Finn needed was a moment of hesitation, and that's exactly what he got. The figure came across a gap that they must have thought was too large to jump, and slowed ever so figure, instead made a sharp left.

"Too slow," Finn shouted at the figure. He dove for the tackle. At that point, everything went in slow motion.

Finn was sailing through the air at the figure who had just made an extremely sharp left at the edge of a rooftop. Contact was made, and they went flying off the roof. Finn wrapped his arms around the figure, as they began their speedy descent towards the ground. The figure let out an extremely girlish scream as they plummeted. Finn realized the mistake he made, but held tightly to the figure. He closed his eyes tightly and waited for the impact, but when it didn't come, he opened his eyes. They were still falling which was weird and it was now dark.

 _Are we, Underground?_ Finn thought. _How did we get here? And who is this?_

Still falling, Finn unlatched on arm from around the figure. He moved his hand to throwback the hood. At this, the figure gained what seemed like a new resolve. The figure reeled back it's head and forcefully brought it forward. Their heads connected.

"OW!" Finn yelled while simultaneously releasing the figure. The figure kicked off Finn, propelling it towards the wall and produced a dagger from under its cloak. The figure proceeded to jam the dagger into the wall and begin to slow its fall. As Finn continued to fall, he caught a glimpse of braided golden hair appear from underneath the cloak.

"What The Hell!" Finn yelled. He hit the ground, and everything went black.

. . .

Weaponry Princess was standing on the balcony in her room still dressed in her nightgown. Just standing and staring and thinking.

 _Where's Finn?_ She thought, _I requested him hours ago._ She wasn't angry, more like annoyed. She had little tolerance for not getting what she wanted almost immediately.

You see there was no real emergency. Actually, the princess of the Weaponry Kingdom seemed to have quite the crush on Finn the Human and just wanted to spend time with him. Being 17 and extremely beautiful, there were many potential suitors for the princess, but she had settled her sights on Finn.

"Oh Well," She said with a sigh, I guess I'll have to see him some other time."

She turned around and began to get ready for her princess duties.


	2. Chapter 2

**Before this chapter begin's, I would like to say thank you to everyone who actually reads this story and sorry it's been so long. This chapters kinda short so sorry about that too. This is my first fanfic so bear with me, and constructive criticism is very much appreciated. I know it's not the perfect story and I'm sorry about that. I would love to become a writer some day so thanks to everyone reading and reviewing and I love you all.**

Chapter Two

Finn bolted back into reality. "Huh?! What the…? Where am I?! Who are You?! Whats going on?!" He said in a half growl, half yell. He tried to move but his arms and legs were bound to the wall. Finn surveyed the room. There was a tall, slender humanoid figure standing in front of him. He couldn't make out any details because it was too dark. He saw his backpack had been thrown in the corner. There were two other men standing behind the taller humanoid.

A light was shone on Finn's face, blinding him momentarily. When his eyes adjusted to the light, he drew in a quick breath as he finally saw the face of the man standing in front of him. His face was upside down and his skin was tinted a faint orange. The long thin nose of the man drooped down between his straight black eyes, and the man's mouth was curved down, what would normally be a frown, but here, it was a smile. This was the first time Finn had seen this in OOO, a creature of any kind that had an upside down face. There were two others standing behind him.

"Ahh, Finn the Human. You've finally awoken." The creepy upside down smile grew wider. "I've been waiting for you to awaken for quite some time. I've heard a lot about you, Finn the Human. The master swordsman, hero of OOO. Saving the day in countless ways.

"Now allow me to introduce myself. I am currently known as Bernard, but my name changes when it needs. As for where you are, you could say," he thought for a second," We are currently… under, the weaponry kingdom.

"Why you are here? Well, it was actually more of a coincidence, a rather pleasant one however. You seemed to have stumbled upon something that you weren't supposed to, and ended up falling right into our clutches. You're going to do great things for us Finn the human. With you here, our plan can commence much more smoothly. Once our plan is complete, Ooo will never be the same."

"What are you planning?!" Finn struggled in his restraints. Finn began to panic a little. What was this big plan? Ooo would never be the same? "Whatever it is, I swear I'll stop you."

"Oh, on the contrary Finn the human. You're going to help us." Bernards smile seemed much more sinister, which was saying something.

"Like hell I will." Finn yelled. He struggled against his restraints, his face contorting into an angry frown.

"I don't think you understand, Finn the human. You don't have a choice." He stepped aside, revealing a large machine. "Finn the human, meet the Hypno-tizer. This machine will change you. It will turn you to our purpose."

"You're gonna brainwash me?" Finn asks. "Been there done that." Finn remembered an adventure he went on a couple months back. An evil witch had controlled his brain and made him do some crazy stuff. Using pure willpower, he was to force the witches control from his head, and beat her up.

"Oh this isn't ordinary brainwashing, Finn the human, this machine alters memories. Your is in the palm of our hands with the Hypno-tizer." He paused to let that sink in. Finn began to panic. This wasn't looking good. "Now, lets get you all strapped up."

After a brief period of struggling and yelling from Finn, bernard was finally able to strap the helmet part of the machine onto Finn's head. The helmet fell above his eyes so he couldn't see anything.

"Stop. You can't do this. I'll stop you. I swear. I'll take this whole operation down."

"Oh Finn, you can't. You're powerless right now." Finn heard a switch flip and a hum. "Now Finn, this is going to hurt. But don't worry, you won't remember it." A very sinister laugh came from bernard, and for the second time in a short time, everything went dark.


End file.
